Recovery of ammonia from exit gases of an ammonium polyphosphate plant

ABSTRACT

AMMONIA CAN BE RECOVERED FROM THE EXIT GASES OF AN AMMONIUM POLYPHOSPHATE PLANT BY SCRUBBING THE REACTOR EXIT GASES WITH PARTIALLY AMMONIATED SUPERPHOSPHORIC ACID HAVING A PH ABOVE ABOUT 3, AND RECYCLING THE PRODUCT OBTAINED THEREBY TO THE REACTOR. IN THEIS WAY THE PRODUCTION OF ORTHOPHOSPHATE IS MINIMIZED.

United States Patent "i e US. Cl. 423-314 4 Claims ABSTRACT OF THEDISCLOSURE Ammonia can be recovered from the exit gases of an ammoniumpolyphosphate plant by scrubbing the reactor exit gases with partiallyammoniated superphosphoric acid having a pH above about 3, and recyclingthe product obtained thereby to the reactor. 'In this way the productionof orthophosphate is minimized.

BACKGROUND OF THE INVENTION Wet process phosphoric acid is manufacturedby treating phosphate rock with sulfuric acid in order to form freephosphoric acid and calcium sulfate. The latter, being insoluble isseparated from the acid by filtration. The wet process acid, as commonlyproduced and handled, is highly corrosive to mild steel at ambienttemperatures and corrosive to most materials including stainless steelsat elevated temperatures. As a result, precautions are necessary inshipping, such as the use of rubber or polyethylene lined containers andstoring the phosphoric acid in lead, brick or rubber lined vessels. Whensuch wet process acid is treated with ammonia to form aqueous ammoniumphosphate solutions, the im-- purities present in the acid formgelatinous precipitates which are substantially impossible to separatefrom the aqueous phase by filtration or other conventional methods. Suchprecipitated impurities in no way interfere with the fertilizing valueof the ammonium phosphate, although they settle in the bottom of storagevessels and clog pipe lines and the equipment used for applying thefertilizer to the soil.

Previous attempts to obtain aqueous solutions of ammonium phosphate fromwet process phosphoric acid having generally been directed to thepurification of the acid. These methods have not been widely acceptedbecause they are complex and costly to perform. They also reduce thenutrient value of the product, since the precipitated impuritiesthemselves are plant nutrients. Thus, because of the aforesaid problemsand disadvantages, substantially all the ammonium phosphate producedfrom wet process phosphoric acid is applied to the soil as fertilizer insolid form. When polyphosphates are used, these impurities do notprecipitate and a substantially clear liquid fertilizer can be produced.Additionally, polyphosphates are substantially completely Water solubleand therefore are substantially completely available as fertilizer. Itis therefore commercially advantageous to market the ammonium phosphatefertilizer in the polyphosphate form.

It has additionally been found that when super-phosphoric acidcontaining non-orthophosphates (polyphosphates) is diluted andthereafter ammoniated at a predetermined rate and a temperature belowabout 210, the resulting product has a high percentage of polyphosphate.For example when concentrated superphosphoric acid is diluted andammoniated at a temperature below about 210 F., the heat provided by theheat of neutralization of the ammoniation process process suflicientheat of evaporation so as to remove a major amount of the water, therebyresulting in a solid high analysis ammonium polyphosphate fertilizermaterial. In order to prevent hydrolysis of the polyphosphates presentin super- 3,687,618 Patented Aug. 29, 1972 phosphoric acid toorthophosphoric acid during dilution, the concentrated superphosphoricacid is diluted just prior to ammoniation at a temperature and rate soas inhibit that hydrolysis.

In that manufacture of ammonium polyphosphates from concentratedsuperphosphoric acid at atmospheric pressure, the reactor exit gases,containing substantial quantities of ammonia, are either vented to theatmosphere or are recovered in a Way which precludes their use in makingadditional ammonium polyphosphate fertilizer. This invention is directedto a method for recovering the ammonia present in reactor exit gaseswith the superphosphoric acid in such a Way that the hydrolysis ofpolyphosphate to orthophosphate is minimized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of thisinvention for pro-- DETAILED DESCRIPTION OF THE INVENTION It has nowbeen found that ammonia can be recovered from the reactor exit gasesproduced in a method for making ammonium polyphosphates by theammonination of superphosphoric acid by scrubbing the reactor exit gaseswith partially ammoniated superphosphoric acid.

With reference to FIG. 1, wherein the process of this invention is shownin a flow diagram, the starting material is concentrated superphosphoricacid containing 60 to by weight phosphorous, expressed as P 0 Thesuperphosphoric acid can be obtained in any convenient way such as byconcentration of wet process acid or from furnace acid. The termsuperphosphoric acid as used defines a phosphoric acid containingsubstantial quantities of polyphosphoric and orthophosphoric acid. Thepolyphosphoric acids include pyrophosphoric acid, and the variouspolymeric acids varying from triple phosphoric acids to metaphosphoricacid and higher polyphosphates. In the practice of this invention it isdesired that the concentrated superphosphoric acid contain as littleorthophosphoric acid as possible in order to maximize the yield ofammonium polyphosphate.

To provide sufiicient Water so that the heat evolved during theammoniation process is removed by the heat required for the evaporationof Water, the superphosphoric acid can be diluted prior to ammoniation.Superphosphoric acid, however, tends to hydrolyze in water to the orthoform and it is therefore necessary to provide a method of preventing orinhibiting hydrolysis and consequent formation of o-rthophosphate. Tothis end, superphosphoric acid is conveniently diluted with water justprior to ammoniation and under conditions of rate and temperature toinhibit hydrolysis of a polyphosphate. In the preparation of ammoniumpolyphosphates a suitable range for dilution of the concentratedsuperphosphoric acid is in the range of from about 49% to about 60%(percent P 0 by weight) superphosphoric acid. Undiluted superphosphoricacid having a concentration (expressed as percent by weight P 0 fromabout 60% to about 80% can be used without dilution if desired. Thesuperphosphoric acid, in convenient concentration, is continually cooledby conventional means to maintain a feed acid temperature of about F.

The diluted superphosphoric acid is passed into the ammoniumpolyphosphate reactor; desirably, it is continuously sprayed therein.

Anhydrous ammonia is continuously passed into the ammonium polyphosphatereactor. Here, the ammonia gas is continuously and thoroughly mixed withthe diluted superphosphoric acid to obtain the ammonium polyphosphateproduct. In a preferred embodiment, the quantity of water in the dilutedsuperphosphoric acid can be such that it is sufficient to maintain thetemperature of the re- Example I This example illustrates theproposition that partially ammoniated superphosphoric acid can absorbammonia without substantial conversion of non-ortho P to the ortho form.

actor bed at a desired level, for example below about 180 5superphosphoric acid was ammoniated with 28% aque F. while also beingvaporized, thereby removing the heat ous ammonia to i a 7 24 0 OIL h Hwas of nfautmhzanon so as to yleld a satlsfactory f maintained above 6.2to prevent the non-ortho P 0 from Qmmum polyphosphafe ProductTheflfoduct converting to the ortho form. Anhydrous ammonia was Fgggfremoved whlle excess ammoma 1S Vented along 10 then passed through the7-240 fertilizer solution to W1 F exlt gasesevaluate its ability toabsorb ammonia. The pH was kept As indicated, the reactor exit gases areStirllbb?d Wlth between 6.2 and 6.5 by the addition of superphosphoriepartially ammomated superphosphoric acid. The exit gases acidTemperature was maintained at betwgen and are desfmbly absorbedscrElbbed the PaY t1 a11Y 160 F. A total of 99 parts of nitrogen waspassed through ammometqd SuPerPhSPhnc f under such condltons thesolution, 88 parts of which were absorbed by the soluas to minimize thehydrolysis of the polyphosphate to tion. Yth9PhSPhaw T it has been lfthe In the course of the initial ammoniation it was found Partlanyammomated Superphosphonc and has a PH that 1.5% of the non-ortho P 0 wasconverted to the above about and Preferably between about 5 and 64 hortho form and the solution absorbed 88.6% of the total hYdYPIYSIS ofthe polyPhosphate to Orthophosphate 1S nitrogen passed through it.During the ammonia absorpsigmficantly reduced. This can be seen fromFIG. 2., where don 02% of the nomortho P205 was converted to the theconversion of non-ortho to ortho P 0 is graphically ortho formrepresented as a function of decreasing pH. As can be Example H i fromthe i at PH below about 3 there IS elm This example illustrates theammoniation of 54% i a f 2 2 23 3 5 2 3322 2; gg g 6 3 2 2 22}: 25superphosphoric acid. 2,252 parts of phosphoric acid were to absorbammonia added gradually to 748 parts of aqueous ammonia to preat a 547acid. The H was maintained above 6.2 b The F temparature 1S Slgmficantand deslri he addition of anhYdIOl l S ammonia gas When all the acidably; mamtamed m the range of from ab-Out Fibout had been added the pHwas allowed to drop to 6.2 and 175 At lower temperatures there 1s apossibil ty of a Sample taken ydrous ammonia was again passedprecipitating polyp 130s? hate material While If the through thesolution and a second sample taken when the temperature 18 mcreasedsignificantly above 175 F. there p reached 6 5 The rate of addition ofsuperphosphoric are attendent corrosion problems and the hydrolysls ofacid was pp 188 parts per hour the anhydrous polyphosphate 1s 1ncreased.In a preferred embodiment, ammonia approximately 26 parts p hourTemperature zg g g ig m the range f g abogt was maintained between 150and 160 F Approximately to a out 1 wi superior resu ts eing o tamedbetween 1500 and 1600 R 2.0% dot thBtII'IIOH-OI'ltllO P 0 was convertedto the ortho The scrubbing solution can have any convenient conorm urmg6 mac centration, but in a preferred embodiment the solution Example Inis diluted. Dilution is required to overcome problems of 40 Twofertilizer solutions having compositions 7-240 viscosity, concentratedsuperphosphoric acid being exand 1344() respectively were tested fortheir ability tremely viscous. In a preferred embodiment the acid is toabsorb ammonia at pH 5.0, 5.5, and 6.2 at temperadiluted to about 50% toabout 60% (P 0 by Weight) to tures of 100 and 150 F. obtain a solutionhaving a viscosity which is convenient to pp y 500 mllllllters offertlllzer solution was work with used for each run. Ammonia was addedat a rate of 0.1 As indicated, the diluted superphosphoric acid ispargamer mmute- The System was kept under Shgh? tially ammoniated. Thiscan be done in any convenient 11m n all efiluent gases passed through asulfuric acid way, the ammoniation advantageously being performed 2:1 aiii sg a gi sg vaggr vigs gggg ig if ig 58; so as to rovide a solutioncontainin from about 5 to 2 about 5, nitrogen by Weight In agpreferredembodi concentration approximately constant. After the ammonia ment theacid is ammoniated rovide from about 5 to absorption, each solution wasanalyzed to determine total p nitrogen absorbed, nitrogen not absorbed,and percent about 10% by weight of nitrogen. Typical solutions Willconversion of nomortho P205 to ortho P2O5 have a Such as 7*24-0: @LPTK)and 1 As will be seen below, the ammonia appeared to be 44-0- At hlgherfieglaees ammomatmn excass Ort absorbed completely by the 7-24-0fertilizer solution un- PhOSPhate formatlm'l 1S Q der all testconditions. There was essentially no conver- In Preferred embodll'nentthe Solution oblalnefl 1111011 sion of the non-ortho P 0 to the orthoform. The 13- Scfubbmg the rfiaQtOr exit gases, Whlch 30100011 15 Char-44-0 solution showed somewhat higher conversion of nonacterized by a loworthophosphate content 15 recycled for ortho P 0 to the ortho form. Theresults are reported hereaction in the ammonium polyphosphate reactor.low and summarized in Table 1.

TABLE 1 Percent Non- Non- Total Grams Percent Final Total ortho ortho]grams GramsN N in Sample pH mp. wt. P505 P405 total N N absorbed trap7-24-0 500.0 24.00 0.53 30. 55 5.05 34.8 A 5.0 100 766.5 28.48 11.4640.24 0. 52.1 17.3 0.0 B- 5.5 708.3 25. 00 10. 20 30. 84 e. 30 51.0 10.20.0 o. 5.2 100 743.3 25.86 10.53 40.72 7.05 52.4 17.0 0.0 1)... 5.0007.0 26.80 10. 01 30.50 5.58 50.6 15.8 0.0 5.5 150 878.0 25. 25 10. 4030. 02 5. 05 52.3 17.5 0.0 5.2 150 2,350.4 20.31 1155 30.44 8.23 103.787.5 0.18 13-444 500 4476 18.34 40.07 13.43 07.2 G- 5.0 100 082.0 40. 5715.85 30.00 8.92 87.7 20.5 0.31 H 5.5 100 825.5 30.54 13.23 33.40 11.8107.5 30.3 0.0 :1. (12 100 714.8 38.80 12.73 32.81 13.41 05.0 28.7 0.01 K5.0 150 871.7 40.52 13.78 34.01 11.08 00.5 20.4 0.0 L 5.5 150 810.0 40.33 13.51 33.45 12.08 07.0 30.7 0.18 10 6.2 150 552.3 42.50 15.21 35.7012.47 81.3 14.1 0.03

5 Example IV This example illustrates the capacity of a 7-24-0fertilizer solution to absorb ammonia at a pH of 2, 3, and 4 at a 150 F.without converting non-ortho P to the ortho form.

A fertilizer solution was prepared having an approximate composition of7240. 300 parts of fertilizer solution was employed in each test. Theammonia was added at a rate of 0.18 part per minute. superphosphoricacid and water were added as necessary to maintain a constant pH andkeep the percent P 0 approximately constant. The ability of the solutionto absorb ammonia was tested at pH 2.0, 3.0, 4.0 at a temperature of 150F. Following ammoniation, solutions were analyzed for percent conversionof non-ortho P 0 and percent nitrogen and the trap was analyzed fornitrogen. It was determined that the ammonia was completely absorbed inall three cases 'but increasing conversion of non-ortho P 0 to or-tho P0 occurred with decreasing pH as follows:

. Percent conversion to ortho form Example V The procedure of Example IVwas repeated using a 14- 49-0 solution and the ability of this solutionto absorb ammonia at pH 2.0, 3.0, 4.0 and 150 F. without convertingnon-ortho P 0 was determined. Results are summarized below.

pH: Percent conversion to ortho form 4.0 3.49 3.0 8.78 2.0 1 1.22

Therefore, I claim:

1. In a method for making ammonium polyphosphates in a reaction zone bythe ammoniation of superphosphoric acid with anhydrous ammonia underconditions adapted to minimize the formation of ammoniumorthophosphates, an improvement for producing ammonium polyphosphate,said improvement comprising scrubbing the reactor exit gases from thereaction zone at a temperature of from about 100 F. to about 175 F. withpartially ammoniated superphosphoric acid containing from about 5% toabout 15% by weight of nitrogen and having a pH above about 3, and belowabout 6 and recycling the product obtained thereby to the reaction zone.

2. A method according to claim 1 in which the partial- 1y ammoniatedsuperphosphoric acid has a pH between about 5 and 6.

3. A method according to claim 1 in which the superphosphoric acid isdiluted to about to by weight P 0 prior to scrubbing.

4. A method according to claim 1 in which the reac tion zone exit gasesare scrubbed with a solution comprising from about 5% to about 10% byweight nitrogen, from about 50% to about 60% by weight P 0 having pHbetween about 5 and 6 at a temperature between about and about B, saidscrubbing being characterized by the minimization of hydrolysis ofpolyphosphate to orthophosphate.

References Cited UNITED STATES PATENTS 3,382,059 5/1968 Getsinger 71-343,562,778 2/1971 Siegel et a1. 71-34 3,171,733 3/1965 Hignett et a171-48 3,192,013 6/1965 Young 23-165 3,310,371 3/1967 Lutz 23-107 OSCARR. VERTIZ, Primary Examiner G. A. HELLER, Assistant Examiner US. Cl.X.R.

